1. Field of the Invention
This invention relates to flow rate measuring devices, and more particularly, to flow rate measuring devices which can be accurately calibrated to yield precise flow rates for Reynolds Numbers at least as high as 30.times.10.sup.6.
2. Description of the Prior Art
Evaluation of the thermodynamic performance of a steam turbine necessitates determining the rate of flow of the motive fluid used therein. Closed loop turbine systems usually include a steam generator element, a turbine element, a condenser element, and a feed pump for returning condensate from the condenser element to the stream generating element. While multiple feed pump elements may be utilized as well as other heat exchanger elements such as regenerative feedwater heaters, flow rate metering devices are commonly inserted in the turbine power plant system between the condenser element and the first feed pump element. The flow rate metering device in such systems is usually a flow nozzle which discharges downstream into a conduit which carries the condensate to the first feed pump. The flow nozzle-conduit combination permits measurement of a pressure differential between the nozzle's condensate influent and effluent flow which, in turn, provides a measurement of the flow rate through the entire closed loop turbine system.
The theoretrical rate of flow for an incompressible fluid such as condensate is dependent upon the square root of the pressure drop through the flow rate meter device. Actual flow rate is related to the theoretical flow rate by a parameter commonly known in flow rate measurement art as the discharge coefficient. Discharge coefficients are usually determined experimentally for flows up to Reynolds Numbers of about 3.times.10.sup.6. Steam turbine systems, however, frequently have Reynolds Numbers of 30.times.10.sup.6 and higher. To date discharge coefficients have not been experimentally determined for Reynolds Numbers larger than 3.times.10.sup.6 because pumps required to produce Reynolds Numbers of such magnitude are not available in calibration laboratories, thus necessitating extrapolation of existing, low Reynolds Number curves to obtain discharge coefficients for flows having Reynolds Numbers larger than 3.times.10.sup.6.
Conventional fluid flow rate meter installations require an overall length of approximately 26 equivalent pipe diameters of straight piping to minimize effects of upstream and downstream elbows, valves and other restrictive elements. A series of multiple hole orifice plates or other conventional flow straightening devices are often installed upstream from the flow rate measuring nozzle to further isolate the nozzle from upstream piping disturbances which introduce swirl and other nonuniform velocity profiles making the nozzle discharge coefficients inaccurate and uncertain. In the ideal situation the discharge coefficient approaches unity, but in conventional flow rate nozzle installations, the velocity of the condensate entering the nozzle is substantial and introduces corner losses at the intersection of the nozzle and attached conduit which make the discharge coefficient artificially low and uncertain. Accurate flow rate measurement requires precise static pressure measurements upstream and downstream from the flow nozzle. It is thus desirable that static pressure measurement exclude any component of dynamic pressure which result from condensate flowing against the static pressure measuring device and registering thereon as static pressure. Many conventional flow rate nozzle installations have upstream static pressure probes which are subjected to substantial fluid velocities and downstream static pressure probes which are customarily at the nozzle's throat where the highest fluid velocity is experienced. Static pressure measurements at the throat of the nozzle have been shown to introduce errors in the required pressure measurement on the order of 1% of the dynamic head. Such errors in the downstream pressure measurement and inaccuracies in the upstream pressure measurement introduce large uncertainties in the pressure drop measurement therebetween which is required in determining the fluid flow rate. Conventional nozzle installations usually permit uncontrolled fluid expansion from the discharge end of the nozzle to the inside diameter of the discharge conduit. Such uncontrolled fluid expansion introduces further large losses in the system's total pressure resulting in a reduction in the turbine system's efficiency. Theoretical discharge coefficients, as required for extrapolation, have never been satisfactorily determined in the conventional nozzle installation because the flow therethrough has not been irrotational nor have the boundary layer characteristics been adequately defined.
Attempts to uniformalize velocity distribution prior to measuring the flow rate include U.S. Pat. Nos. 3,733,898 issued May 22, 1973, and 3,374,673 issued Mar. 26, 1968. U.S. Pat. No. 3,733,898 constitutes a vortex regulator, a flow straightener, and flow converting means which uniformalize vortices, straighten the resulting vortices, and provide a uniform velocity distribution across the diameter of the conduit respectively. While one of the embodiments of the aforementioned patent illustrates a conduit portion of increased cross-section, that portion is too small to constitute a plenum chamber. A true plenum chamber would obviate the need for the vortex uniformalizer, the vortex distributor, and the velocity uniformalizer since, prior to the fluid entering the flow rate measurement nozzle, the fluid is brought to rest or substantially so thus performing all the functions of U.S. Pat. No. 3,733,898 in a shorter flow distance and much simpler construction. U.S. Pat. No. 3,374,673 includes a structure having an increased cross-sectional flow area which permits fluid to flow through a foraminated body without suffering an appreciable pressure loss.
The prior art suffers from an inability to extrapolate discharge coefficients with any substantial certainty beyond Reynolds Numbers of approximately 3.times.10.sup.6. Such inability results from a combination of causes including lack of a boundary layer theory which accurately and satisfactorily predicts fluid behavior prior to its entry into the differential pressure flow rate nozzle. The inability to accurately measure static pressure upstream and downstream from the flow nozzle, extremely long, unrestricted flow conduits for the flow metering nozzle, and large pressure losses constitute further disadvantages of the prior art.